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American Astronomical Society, Astrophysical Journal, 1(799), p. 86, 2015

DOI: 10.1088/0004-637x/799/1/86

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The Spectrum of Isotropic Diffuse Gamma-Ray Emission between 100 MeV and 820 GeV

Journal article published in 2014 by Fermi Lat Collaboration, Markus Ackermann ORCID, M. Ajello ORCID, A. Albert, W. B. Atwood, L. Baldini ORCID, J. Ballet ORCID, G. Barbiellini, D. Bastieri ORCID, K. Bechtol, R. Bellazzini, E. Bissaldi ORCID, R. D. Blandford, E. D. Bloom, E. Bottacini and other authors.
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

The {γ}-ray sky can be decomposed into individually detected sources, diffuse emission attributed to the interactions of Galactic cosmic rays with gas and radiation fields, and a residual all-sky emission component commonly called the isotropic diffuse {γ}-ray background (IGRB). The IGRB comprises all extragalactic emissions too faint or too diffuse to be resolved in a given survey, as well as any residual Galactic foregrounds that are approximately isotropic. The first IGRB measurement with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey data and considered an energy range between 200 MeV and 100 GeV. Improvements in event selection and characterization of cosmic-ray backgrounds, better understanding of the diffuse Galactic emission, and a longer data accumulation of 50 months, allow for a refinement and extension of the IGRB measurement with the LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB spectrum shows a significant high-energy cutoff feature, and can be well described over nearly four decades in energy by a power law with exponential cutoff having a spectral index of $2.32±0.02$ and a break energy of $(279±52)$ GeV using our baseline diffuse Galactic emission model. The total intensity attributed to the IGRB is $(7.2±0.6) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ above 100 MeV, with an additional $+15$%/$-30$% systematic uncertainty due to the Galactic diffuse foregrounds.